Abstract: Antimony (Sb) contamination in mining area soils is severe, and its environmental risk mainly depends on the chemical speciation of Sb. Mn oxides and humic acids are key active components that regulate Sb speciation in soil. At present, most of the research focuses on the single adsorption behavior toward Sb, but the interaction between them is less studied, especially in real soil environments. Using an in situ preparation method, 0.1% birnessite (δ-MnO₂) was loaded on Sb-heavily contaminated yellow soil in the mining area. Humic acid was added at 1.0%, 2.0%, 3.0%, and 5.0% of the soil mass. After 40 days of flooding, the changes of various forms of Mn and Sb in the soil were analyzed, and the influence mechanism of the interaction between humic acid and δ-MnO₂ on the transformation of Sb species in the soil was explored. The results indicated that with increase of humic acid addition, the content of suspended Sb and weakly acid-extractable Sb in the original soil group decreased by 20.7 times and 75.8%, respectively. Meanwhile, the content of reducible Sb, oxidizable Sb, and residual Sb increased by 75.4%, 41.0%, and 4.6%, respectively. In the Mn-loaded soil group, suspended Sb and weakly acid-extractable Sb decreased by 41.2 times and 84.6 times, respectively, while reducible Sb, oxidizable Sb, and residue Sb increased by 83.2%, 2.9 times, and 7.9 times, respectively. Compared with the original soil, the contents of weakly acid-extractable Sb and reducible Sb in Mn-loaded soil increased by 19.4%–36.7% and 41.3%–77.6%, respectively. When humic acid content was greater than 3.0%, the content of oxidizable Sb increased by 26.5%–38.2%. Humic acid and δ-MnO₂ can synergistically convert highly mobile suspended Sb and weakly acid-extractable Sb in soil into more stable reducible, oxidizable, and residual forms, thereby reducing Sb migration. These findings provide critical theoretical support and a practical pathway for the synergistic remediation of Sb-contaminated soils using Mn oxides and humic acid. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202509020229.